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parasites hijack their host's brains

Neuroscience For Kids

How do Parasites Hijack their Host's Brains?
The Neuroscience of Toxoplasmosis
By Kristin Harper, Neuroscience for Kids Guest Writer

October 11, 2013

A dog with rabies suddenly feels the urge to bite other animals, passing
along a virus in the process. A cricket infected with a horsehair worm
will seek out water and drown itself, ensuring that its unwanted passenger
gets the chance to mate. And when a parasitic ant fungus is ready to
release its spores, it somehow convinces its tiny insect host to climb a
plant and bite the underside of a leaf. The ant dies, fungal fruiting
bodies emerge from its body, and the cycle of life continues for the
fungus.

How can we explain these strange behaviors? Evolution has ensured that
viruses, bacteria, and parasites have perfected the art of infection-even
changing the way their hosts behave to ensure that they, the infectious
hitchhikers, are able to reproduce. This is called "behavioral
manipulation," and in most cases, scientists know very little about how a
pathogen affects its host's brain, forcing it to do such odd things.

Toxoplasmosis gondii: A cat-and-mouse parasite

Recently, researchers have begun to uncover the pathways through which
a parasitic protozoan called Toxoplasmosis gondii can alter
behavior. T. gondii causes the disease toxoplasmosis in humans, but
its primary host is the cat. Cats typically get infected by eating rodents
infected with T. gondii. The rodents become infected after eating
food contaminated by the feces of infected cats. In order to keep the
wheel of transmission rolling, T. gondii forces its unlucky rodent
hosts to do things they would normally find terrifying. For example, mice
and rats typically avoid the smell of cat urine. This makes perfect
sense-it helps them avoid cats, their natural predator. When these animals
become infected with T. gondii, though, they suddenly become
attracted to the smell of cat urine! This works out very well for the
parasite, because once a cat catches and eats an infected rodent, the next
generation of T. gondii can be produced.

How does Toxoplasma make the smell of cat urine appealing?

After infecting its rodent host for a few weeks, the parasite forms
cysts in the animal's brain. These cysts display a slight preference for
the limbic system regions, the areas of the brain that govern both
predator avoidance and attraction to the opposite sex. The pathways in the
brain that help control these two behaviors are separate, but they lie
very close to one another, running in parallel through the medial amygdala
and hypothalamus.

When an infected rat is exposed to cat urine, studies show that neural
activity shifts from the animal's predator avoidance pathway to the nearby
attraction pathway. This pathway mismatch leads an infected rat to become
aroused, causing it to spend a lot of time around the scent of cat urine
instead of being repelled by the dangerous odor. In this case, evolution
has hit upon an ingenious way for T. gondii to exploit the rat's
brain circuitry.

Female rats are more attracted to infected males

Normally,
female rats try to choose mates that are parasite-free. In the case of
T. gondii, this makes a lot of sense for two reasons. First,
because T. gondii is transmitted between rats during mating, a
female that rejects infected males can avoid infection herself. Second, a
parasite-free father is more likely to sire parasite-free offspring who
can avoid fates like the one described above.

But what would happen if female rats consistently rejected T.
gondii-infected males? Infection levels would drop, and over time rats
would become more and more resistant to the parasite. This would be bad
news for T. gondii. But the parasite appears to have taken care of
this threat by manipulating the way female rats choose their mates.
Instead of repulsing choosy females, males infected with T. gondii
become more attractive.

Even the scent of an infected male's soiled bedding is more appealing
to females than the scent of an uninfected male's bedding. It appears that
the parasite is somehow capitalizing on the role that scent plays in
attraction. The specific mechanism by which T. gondii dupes females
into falling for parasitized partners remains unknown.

Humans get toxoplasmosis too

It is not just cats and rats that
need to worry about T. gondii. Humans can become infected by eating
contaminated meat or via contact with feces from an infected cat. T.
gondii is dangerous to babies before they are born, too. That is why
pregnant women are told not to clean out the cat litter box at home!
Roughly 30% of the world's population carries the parasite. That's one out
of every three people worldwide. An infected mom can pass the infection
on to her baby who may develop serious symptoms later.

Some researchers have become convinced that T. gondii
manipulates our behavior as well. Some odd findings have been
reported:

People with latent toxoplasmosis (those who are not showing
symptoms but carry the parasite) are more likely to get in traffic
accidents.

Infected women are more likely to attempt suicide.

Infected men find the smell of cat urine more attractive.

People who have schizophrenia are more likely to be infected with
toxoplasmosis than those who do not. This has led some scientists to
believe the parasite may be an important risk factor for this mental
illness.

Is the parasite controlling our behavior, just as it
controls that of rats? It is unclear what the mechanism is behind all of
these odd behaviors, but some scientists believe an increase in levels of
dopamine (a neurotransmitter) may be the cause. Why? It turns out that the
brains of T. gondii-infected mice have elevated levels of this
chemical, and dopamine is also believed to play an important role in
schizophrenia. Future studies should clarify just how much toxoplasmosis
influences our behavior as well as which changes in the brain are
responsible.

Research on how the infection affects mice and rats, while interesting
in its own right, may provide valuable insights into the mechanisms at
work in humans as well. Someday, we may even be able to exploit the
information we learn about how this parasite manipulates its hosts' brains
to deliver new treatments to patients suffering from brain-based
disorders.

Did you know?

Toxoplasmosis is considered to be a leading cause of death attributed
to foodborne illness in the United States. Symptoms of toxoplasmosis
include flu-like symptoms: fever, tiredness, sore muscle, sore throat, and
headaches. The parasite can cause the tissue in the intestine and lymph
nodes to die. Other body parts can be affected, too, such as the eye,
heart, and adrenal glands. Dehydration, coma, and death may even occur.
Additionally, if a person's immune system is not strong, meningitis, an
infection of the brain, can occur. More than 60 million men, women, and
children in the U.S. carry the Toxoplasma parasite, but very few have
symptoms because the immune system usually keeps the parasite from causing
illness. (Source: Centers for Disease Control and Prevention -
http://www.cdc.gov/parasites/toxoplasmosis/)

Kristin Harper is a freelance science writer and editor based in New
York. She has a Master's degree in Public Health (MPH) in Global
Epidemiology and a PhD in Population Biology, Ecology, and Evolution. She
has always been interested in psychiatric disorders and the evolution of
pathogens, so she is fascinated by conditions, like Toxoplasma infection,
in which the two intersect.